Reconnaissance study of mylonitic fabrics in the Bellota Ranch area, eastern Santa Catalina Mountains, Arizona

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Author
Spencer, J.E
Constenius, K.N.
Affiliation
Arizona Geological Survey
Issue Date
2020
Keywords
Arizona Geological Survey Contributed Reports
Metamorphic
core complex
detachment fault
gneiss
mylonite
Bellota Ranch
Molino Basin
Pima County
Rincon Mountains

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Citation
Spencer, J.E. and Constenius, K.N., 2020, Reconnaissance study of mylonitic fabrics in the Bellota Ranch area, eastern Santa Catalina Mountains, Arizona. Arizona Geological Survey Contributed Report CR-20-C, 12 p, one appendix.
Publisher
Arizona Geological Survey (Tucson, AZ)
Description
The Santa Catalina Mountains in southeastern Arizona include extensive mylonitic fabrics developed within granitic and gneissic rocks that make up most of the range. These fabrics are strongest at the southern foot of the range where they dip south and project beneath the rangebounding Catalina – San Pedro detachment fault. Shear sense in the mylonitic rocks is primarily top-southwest, consistent with shearing down-dip from the detachment fault during early normal faulting and exhumation to form the metamorphic core complex. Two to three kilometers north of the foot of the Santa Catalina Mountains the mylonitic foliation is horizontal and farther north it dips to the north. The mylonitic fabrics thus dip outward from the axis of the Forerange arch with primarily top-northeast shear-sense indicators on the north side of the arch. This dominantly topnortheast fabric forms the Molino Basin shear zone, which continues eastward toward the Bellota Ranch area that is the subject of this study. We found that mylonitic fabrics in the Bellota Ranch area are generally subhorizontal, weak, and without clear shear-sense indicators in by far ost outcrops. Four days of field study yielded clear shear-sense indicators at only eight outcrops, with six of them indicating top-southwest shear sense. We were not able to divide mylonitic rocks into top-southwest and top-northeast zones as in a previous study by Bykerk-Kauffman (2008). Lineation trend gradually changes laterally from more northeasterly in the Molino Basin area to more northerly in the Bellota Ranch area. As with the Molino Basin area, lineations associated with top-northeast shearing generally trend more northerly than lineations associated with top-southwest shearing. We conclude that mylonitic lineation in the Bellota anch area is not a simple continuation of the Molino Basin shear zone as it is dominated by top-southwest shear-sense indicators rather than top-northeast as with the Molino Basin shear zone.
URI
http://hdl.handle.net/10150/642094
Additional Links
https://library.azgs.arizona.edu/
Language
en
Series/Report no.
CR-20-C
Rights
Arizona Geological Survey. All rights reserved.
Collection Information
Documents in the AZGS Documents Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu.
North Bounding Coordinate
32.5168
South Bounding Coordinate
32.3476
West Bounding Coordinate
-110.786
East Bounding Coordinate
-110.556
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